Borg Warner BW-35 Transmission Fluid Guide

The Borg-Warner 35 transmission, often abbreviated as BW-35, is a well-regarded automatic transmission that found its way into a wide array of vehicles from the 1960s through the early 1970s. This robust unit, along with its close relatives the M-36 and M-37, offered a smooth and reliable driving experience for many. If you're the proud owner of a vehicle equipped with one of these transmissions, understanding its specific fluid requirements is paramount for maintaining its longevity and performance. This comprehensive guide will delve into the crucial details of transmission fluid for the Borg-Warner M-3x series, ensuring your gearbox continues to operate at its best.

Understanding the Borg-Warner M-3x Series

The "3" in the Borg-Warner 35 designation signifies its place within a broader family of aluminium-cased automatic transmissions. These include the M-4x, M-5x, and M-6x series, all sharing a common lineage stemming from the original M-35. While generally, a higher series number indicated increased strength and torque handling capability, this wasn't a universal rule across all Borg-Warner automatics. For instance, earlier cast-iron cased transmissions like the M-8 and M-1x were often stronger than their aluminium counterparts, though the M-6x series could rival the M-1x in power handling.

The second digit in the model number typically denotes a specific variation, often related to enhanced torque load capacity or other refinements. The M-3x transmission, in its standard configuration, features three forward gears and one reverse gear. The selector lever and quadrant layout, however, could vary depending on the vehicle manufacturer and the specific production year.

Selector Quadrant Variations

Early M-3x transmissions often featured a six-station quadrant with positions marked P-R-N-D2-D1-L (Park, Reverse, Neutral, Drive 2, Drive 1, and Lock). In the 'D2' position, the transmission would start in second gear, offering improved fuel economy on relatively flat terrain and better traction on slippery surfaces like mud, snow, or ice. Selecting 'D1' would engage all three forward gears. The 'L' (Lock) position prevented upward gear changes, providing maximum engine braking in first gear and moderate braking in second. This feature allowed for strategic gear selection, particularly useful for downhill driving or maintaining control in adverse conditions.

From 1965 onwards, a more familiar P-R-N-D-2-1 shift pattern became common. AMC, for example, marketed this as "Shift-Command." This evolution simplified operation for many drivers who expected automatic transmissions to cycle through all forward gears in the primary drive position.

M-36 and M-37: Enhanced Capabilities

The M-36, introduced in 1965, was largely identical to the M-35 but included provisions for an external transmission oil cooler. While the M-35 relied on the torque converter's fan for air cooling, the M-36's cooler made it suitable for heavier vehicles or those used for towing. The M-37, first documented in 1967 AMC Technical Service Manuals, offered an even higher torque rating and was often used in larger AMC vehicles, replacing the M-36 behind more powerful engines.

Transmission Fluid: The Lifeblood of Your BW-35

The correct transmission fluid is absolutely critical for the proper functioning and longevity of your Borg-Warner automatic transmission. Using the wrong type of fluid can lead to shifting problems, increased wear, and potentially severe internal damage. Fortunately, for the BW-35 and its related models, the fluid requirement is generally consistent.

What Type of Fluid is Required?

The Borg-Warner BW-35, M-36, and M-37 transmissions were designed to operate using Dexron-II or a fluid that meets the specifications of Dexron-II. In many cases, a quality Type A Suffix A (TASA) automatic transmission fluid (ATF) may also be compatible, especially in earlier models. However, as Dexron-II became the industry standard, it is the most commonly recommended fluid for these units.

It is crucial to note that modern ATFs, such as Dexron-VI, are generally not backwards compatible with older transmissions like the BW-35. Dexron-VI has different friction modifiers and viscosity characteristics that can negatively impact the performance and durability of these older gearboxes. Always opt for a fluid specifically labelled as compatible with Dexron-II or TASA, or a universal ATF that explicitly states compatibility with these older specifications.

Fluid Capacity

The typical fluid capacity for a Borg-Warner BW-35 transmission, including the torque converter, is approximately 8 to 9 Imperial Pints (4.5 to 5 Litres). However, this can vary slightly depending on the specific application and the torque converter size. It's always best to check your vehicle's owner's manual or a reliable service manual for the exact capacity.

When refilling the transmission, it's important not to overfill it. Overfilling can lead to foaming of the fluid, reduced efficiency, and potential seal damage. Add fluid gradually and check the level according to the procedure outlined below.

Checking and Maintaining Transmission Fluid Level

Regularly checking the transmission fluid level is one of the simplest yet most effective maintenance tasks you can perform. This allows you to catch potential leaks or fluid degradation early on.

The Engine Running vs. Engine Off Debate

A point of discussion among mechanics and enthusiasts regarding the BW-35 and similar transmissions revolves around the correct procedure for checking the fluid level: with the engine running or with the engine off. The original Borg-Warner service manuals often specified checking the fluid with the engine switched off. However, many modern mechanics and even some later service updates (like those from Ford Australia, which used these transmissions) recommended checking it with the engine running, typically within seconds of shutting it off or with the engine idling in Park.

Why the discrepancy? When the engine is running, the torque converter is filled with fluid, and the transmission's internal pump is circulating it. Checking with the engine running provides a more accurate representation of the fluid level during operation. Conversely, checking with the engine off assumes the fluid has drained back into the sump, which might not always be entirely accurate due to the nature of torque converters.

Given that many transmissions were serviced with the engine running by experienced mechanics, and some manufacturers adopted this method, it's generally considered more reliable. However, if your specific vehicle's manual dictates checking with the engine off, it's wise to adhere to that. If unsure, checking both ways after the fluid has had time to settle (engine off) and then with the engine running (in Park) can give you a good understanding of the fluid's behaviour.

Step-by-Step Fluid Check (Engine Running Method Recommended)

1. Ensure the vehicle is on a level surface.
2. Start the engine and allow it to reach normal operating temperature.
3. With the transmission in Park (P) and the parking brake firmly applied, move the selector lever through each gear position, pausing for a few seconds in each, before returning it to Park.
4. Locate the transmission dipstick. It's usually a yellow or red metal loop handle, often located towards the rear of the engine on the passenger side.
5. Remove the dipstick and wipe it clean with a lint-free cloth.
6. Reinsert the dipstick fully, then remove it again.
7. Check the fluid level against the markings on the dipstick. There are typically 'HOT' and 'COLD' markings, or 'ADD' and 'FULL' indicators.
8. The fluid level should be between the 'ADD' and 'FULL' marks when the transmission is hot and the engine is running.
9. If the level is low, add the correct type of transmission fluid (Dexron-II or equivalent) a little at a time, rechecking the level frequently, until it reaches the correct mark. Do not overfill.
10. Inspect the fluid's condition. It should be reddish or clear and have a clean, slightly sweet smell. If the fluid is brown, burnt-smelling, or contains particles, it indicates a potential problem that requires further investigation and likely a fluid and filter change.

Common Issues and Maintenance Tips

While the BW-35 is a durable transmission, like any mechanical component, it requires proper care.

Fluid and Filter Changes

A regular fluid and filter change is essential. The general recommendation is to change the fluid and filter every 25,000 to 50,000 miles (40,000 to 80,000 km), or as specified in your vehicle's service manual. This service removes accumulated contaminants and ensures the transmission is filled with fresh, clean fluid.

The process involves draining the old fluid from the transmission pan, removing the pan, replacing the filter, cleaning the pan, and refilling with the correct amount of new fluid. It's a job that can be undertaken by a competent DIY mechanic, but professional servicing is also readily available.

Band Adjustments

Some owners, like the individual asking about a 1974 Toyota Crown, may inquire about band adjustments. The BW-35 does have adjustable bands that control the engagement of certain gear sets. Over time, these bands can stretch, requiring adjustment to ensure proper shifting. If you experience slipping or delayed engagement, a band adjustment might be necessary. This is a more technical procedure and is often best left to experienced transmission specialists, as incorrect adjustment can lead to severe damage.

Cooling System

For vehicles equipped with the M-36 or an aftermarket cooler, it's important to ensure the cooler lines are intact and free from leaks. Overheating is a major enemy of automatic transmissions, and a properly functioning cooling system is vital, especially when towing or operating in hot climates.

Troubleshooting Common Problems

Slipping Gears: This can be caused by low fluid level, worn clutch packs, or incorrectly adjusted bands. Check the fluid level first, then consider band adjustments or a more in-depth inspection.

Harsh Shifting: Low fluid pressure, a faulty valve body, or incorrect fluid type can lead to harsh shifts. Ensure the correct fluid is used and the fluid level is appropriate.

Delayed Engagement: Similar to slipping, this can be due to fluid level issues, a worn pump, or band problems.

Fluid Leaks: Regularly inspect the transmission pan gasket, cooler lines (if applicable), and seals for any signs of leakage. Address leaks promptly to prevent fluid loss.

Frequently Asked Questions

Q1: Can I use Dexron-VI in my Borg Warner BW-35?
A1: No, it is strongly advised against using Dexron-VI. Its formulation is different and can harm older transmissions. Stick to Dexron-II or TASA compatible fluids.

Q2: How often should I change the transmission fluid?
A2: Generally, every 25,000-50,000 miles (40,000-80,000 km) is a good interval, but consult your vehicle's service manual for the manufacturer's recommendation.

Q3: My transmission fluid smells burnt. What does this mean?
A3: Burnt-smelling fluid indicates overheating and excessive friction, likely due to worn internal components or a lack of lubrication. A fluid change might be a temporary fix, but a professional inspection is highly recommended.

Q4: Is it normal for the fluid level to be low?
A4: No, a consistently low fluid level usually indicates a leak that needs to be addressed immediately.

Q5: Can I adjust the bands myself?
A5: While possible, it requires precision and knowledge. If you're not experienced, it's safer to have a professional perform band adjustments to avoid damaging the transmission.

Conclusion

The Borg-Warner BW-35, M-36, and M-37 transmissions are a testament to durable automotive engineering. By understanding their specific fluid requirements and adhering to a regular maintenance schedule, you can ensure these reliable gearboxes continue to serve you well for many years to come. Always prioritize using the correct Dexron-II or equivalent fluid, maintain the proper fluid level, and address any signs of trouble promptly. Proper care will undoubtedly lead to a longer, more trouble-free life for your classic automatic transmission.